Extract the 'postprocess' and the 'tracks' folder to the main directory of DiRT Rally (by default [X]:\steam\steamapps\common\DiRT Rally) and replace/overwrite the existing files.

[2] Go to DiRT Rally\tracks\locations\germany\panzerplatte_rally_01

Inside this directory, you will find the 'route_x' folder that was extracted, amongst others, in step [1]. Copy all 4 files inside this folder (sky_day, sky_day_alt, terrain_lm_day & terrain_lm_wet.pssg) and paste them into the following 6 folders usually to be found directly above the 'route_x' folder:

route_0
route_1
route_2
route_3
route_4
route_5

Again, replace the existing files.

The 'route_x' folder can be deleted afterwards.

[3] Go to DiRT Rally\tracks\locations\germany\panzerplatte_rally_02

Repeat the same procedure from step [2] with the files of the 'route_x' folder found inside this directory, so just copy the files and paste them into the remaining 'route (0-5)'-folders accordingly. Replace any existing files.

[4 - OPTIONAL, BUT RECOMMENDED]

Extract the contents of the 'RESHADE' folder (dxgi.dll, Reshade.fx & ReShade folder) inside this zipped archive to the main directory of DiRT Rally.

The Reshade framework provides additional color correction features (mainly sepia and gamma correction) at almost no additional performance cost. All credits for this tool go to crosire, the creator of ReShade. The pictures were taken with ReShade turned on.

FAQ

Q: I want to upload this mod somewhere else, is this this possible?
A: Please contact me first!

Q: Are you going to improve the lighting of other locations as well?
A: Who knows, let's wait and see

Q: Which tools did you use?
I used Fergo's BinXML editor for .xml files, the EGO PSSG Editor for the .pssg files and of course Photoshop and Paint.net for tweaking the textures. Screenshots were made with fraps and with the ReShade framework turned on.

The major part of the mod that merely replaces textures and .xml files should have no impact on your fps, the optional reshade framework can cost between 1-3 fps depending on system and ingame situation.
Thanks for your review btw.!

Would love for you to make this into a pack for all locations! They all seem to have a their own unique colourfilter or whatever it is. Like Wales is grey/greenish, Greece reddish.. etc. I respect the dev's art vision but I myself prefer neutral colours.

Build me a time machine and I'll do it ;)
Seriously, modding this way takes up huge amounts of time, so as much as I'd love to have a perfectly looking game myself, I'm afraid I won't be able to cover all locations...